![]() Here's a link to a recent SE Chem question: How can I deduce the linearity of XeF2 from the IR spectrum? where these rules were used to determine the structure of a molecule. So for carbon dioxide there is 1 Raman band and two IR bands. the rule of mutual exclusion, it states that, for centrosymmetric molecules (molecules with a center of symmetry, like carbon dioxide), vibrations that are IR active are Raman inactive, and vice versa.Leave "polar" out of the criteria for ir activity and stick with dipole moment, it is a much better understood term. The terms "polar" and "non-polar" can be confusing, they often mean different things to different people. The initial dipole moment in the molecule's equilibrium geometry can be zero all you need is a change. An ir active band will be observed if a vibration results in a change of the dipole moment.The number of molecular vibrational modes equals 3n-6 (3n-5 for linear molecules), where n is the number of atoms.That’s why we discuss above that if we want to judge the molecule polar or nonpolar, we determine the net dipole moment. In CO2, the individual dipole moment exists. So, carbon dioxide is a nonpolar molecule. ![]() The first 3 rules you learn for interpreting IR and Raman spectra are In the case of carbon dioxide, the dipole moment is from carbon to oxygen but the value of the net dipole moment is zero. Either the author 1) inadvertently switched the column headings (IR active, IR inactive) or 2) meant to use some molecule other than carbon dioxide. To sum up, carbon dioxide has 2 ir-active vibrations.Įdit - response to example added (question d) by OP But these two motions are the same, just deforming in different directions, the bend is said to be degenerate, accounting for the "fourth" vibration. This is because the "bend" (let's start by placing the molecule along the x-axis) can occur in the y direction and the z direction. We expected 4 vibrations and I've only listed 3. The bend also results in a change in dipole moment so it too is ir-active.The asymmetric stretch does result in a change in dipole moment so it is ir-active. ![]() Zero dipole moment), so it is ir-inactive.
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